The invention relates to a method for manufacturing a thread-forming screw and a stamping device for carrying out the method.
Thread-forming screws have a shank, which is provided, at least region-wise, with a thread as an external thread, and with a screw head at one end, for example, serving as a load application means To permit thread-forming or thread-cutting screws to form a counter-thread in the substrate, the thread is hardened region-wise, for example, so as to increase the strength. For exterior applications, screws, such as concrete screws, are also manufactured of corrosion-resistant steel materials whose strength normally cannot be increased enough by heat treatment to allow a secure cutting into concrete.
A thread-forming screw of a metal is known from European Patent Document No. EP 1 595 080 B1 where cutting elements of a metal having a higher carbon content are welded on. Therefore, the cutting elements have a hardness that is greater than the hardness of the screw or the thread, respectively. In welded-on state, the cutting elements have a small projection with respect to the outside contour of the thread.
The disadvantage of the known solution is that for welding on the cutting elements, the base material, at least of the thread or the screw, respectively, in this region, must be sufficiently softened so that the cutting element can be pressed into the thread. According to EP 1 595 080 B1, this is the only way that the cutting element can be sufficiently anchored in the thread or the core, respectively, and an excessive projection beyond the outside contour of the thread can be prevented. As a result of the long process time required for this, the screw material is locally brought to high heat which alters the structure of the base material and may lead to the formation of cracks in the material.
In the fixing region the corrosion resistance of the shank may decrease region-wise due to the action of the heat.
The object of the invention is to create a method for manufacturing a thread-forming screw, especially of a corrosion-resistant material, with welded-in cutting elements, which prevents the above-mentioned disadvantages and allows a simple manufacturing of the screw.
According to the invention, the method for manufacturing a thread-forming screw having a shank and at least one thread formed in one piece with the shank and at least region-wise circumferentially arranged on said shank comprises the following steps.
First, the thread is formed on the shank, for example by using a rolling process. Then, a plurality of recesses or indentations is stamped into the thread, wherein the material of the screw is hardened in this region, at least in part. Then, a plurality of compact cutting elements is welded into the previously generated recesses in the thread. The cutting elements are made of a hard material and have a hardness greater than the hardness of the thread.
The cutting elements have an extent following the course of the thread, for example, which corresponds to the extent of the corresponding recesses in the thread. As a result of the arrangement of the cutting elements in recesses in the thread, the base material of the thread or the screw, respectively, is melted to a lesser degree, and therefore, the corrosion resistance of the screw is also largely preserved in the fixing region of the cutting elements. The recess for receiving a cutting element extends, for example, in the direction of the course of the thread, which substantially corresponds to the corresponding extent of the cutting element in its arranged state in the thread.
The recesses are generated in the thread during the stamping in only one forming step, wherein the generated recess or indentation may reach into the core of the screw. The recesses are configured point-shaped or line-shaped, for example. In this context, point-shaped means a recess whose longitudinal extent along the thread corresponds to a maximum of 1.5 times the width extent of the thread on the thread bottom, i.e., on the outside of the shank.
Line-shaped in this context means a recess having a longitudinal extent along the thread of more than 1.5 times the width extent of the thread on the thread bottom, i.e., on the outside of the shank.
Due to the welding into the recesses, the compact cutting elements are so firmly bonded with the thread that they are sufficiently held in the screw, both in the screwing-in and in counter screwing-in direction of the thread-forming screw as well as radially. The wearing of the thread of the screw that is formed in one piece with the shank is reduced even with hard mineral substrates, such as high-strength types of concrete, compared to a thread-forming screw without a cutting element, and the thread-forming process during the setting of the thread-forming screw is improved. In spite of the advantageous setting behavior, the corrosion resistance of the screw is largely preserved, and because of the low wear of the thread higher load ratings are achieved for a set screw compared to existing systems. Furthermore, the core requires no additional processing, which likewise has a favorable effect, for example, on the corrosion resistance and the cost-effective manufacture of the thread-forming screw. The screw is advantageously made of an austenitic corrosion-resistant steel.
The cutting elements advantageously already have a cutting geometry that corresponds to their function and carry out this function immediately after being fixed in the thread. A complex subsequent mechanical finishing of the cutting elements welded into the thread, for example by grinding, milling or hardening, is not required. The hardness of the boundary layer of the cutting elements and thus the advantageous thread-forming properties of the cutting elements are preserved.
The form of the cutting elements and their number are freely selectable and advantageously based on the effect to be achieved. For example, the cutting elements have a sphere-like, cylindrical, rectangular, conical, or pyramidal form. The cutting elements can also be configured as a so-called cutting tooth, such as used for demolition and mining. In order to additionally influence the thread-forming behavior of the thread-forming screw, the cutting elements can also be arranged inclined relative to a tangent applied to the outside contour of the thread. The angle of inclination is 5° to 15°, for example.
The number of cutting elements arranged in the thread depends on the length of the thread and the grooving to be completed as well as the substrate material, in which the thread-forming screw is to be set. According to an advantageous embodiment of the thread-forming screw of the invention, only in one region starting from the setting direction end, a certain number, for example, from four to fifteen cutting elements are provided in the thread spaced apart from each other, while the rest of the thread has no cutting elements. In order to achieve certain setting properties, cutting elements of different configurations may also be arranged on a thread-forming screw.
Advantageously, the fixed cutting elements harmonically continue the thread, at least region-wise, which results in advantageous grooving properties for the screw and a low wearing of the thread. Alternatively, the fixed cutting elements project at least region-wise radially outward beyond the thread, which generates strong grooving and thus increases the removal of the grooved substrate.
In an arrangement of a plurality of cutting means along the course of the thread, for example, the cutting elements fixed in the recesses by means of welding and facing the setting direction end project radially and the following cutting elements decrease continuously in their radial projection until their radial projection corresponds to the radial extent of the thread and the cutting elements thus harmonically continue the thread.
In addition to the compact cutting elements of hard material, cutting elements made of a weld metal, for example, according to a metal gas-shielded metal-arc welding process or by laser cladding, can also be arranged on the thread. Depending on the requirements, these additional cutting means are advantageously also arranged in previously stamped recesses in the thread. The additional cutting means support the compact cutting elements of a hard metal that are welded into the recesses in the grooving of the counter-thread in the substrate. For example, the cutting elements made of a weld metal are provided on the thread alternating with the compact cutting elements of a hard material. Alternatively, in the region of the thread which first comes into contact with the substrate, compact cutting elements of a hard material are arranged in recesses followed by cutting elements of a weld metal arranged in recesses or directly on the thread. According to another alternative, depending on the hardness of the arranged cutting elements, cutting elements of a weld metal followed by compact cutting elements of a hard material can be arranged in the thread in the region of the thread which first comes into contact with the substrate.
The stamping of the recesses and the welding in of the compact cutting elements are preferably directly interlinked, and therefore, the screw does not have to be readjusted for the welding of the cutting elements into the recesses. For example, during the stamping of the recesses, the screw is held by a manipulator, subsequently moved by the manipulator to the welding station and held by it during the welding process.
Preferably, a plurality of recesses is stamped into the thread at the same time. This can increase the process speed and increases the life of the tool, for example of a stamping die.
A plurality of successive recesses is preferably stamped at a distance from each other. The distance increases starting at one free end region of the shank along the course of the thread. In the region of the thread which first comes into contact with the substrate when the thread-forming screw is set, the cutting elements are advantageously arranged close to each other. In a section opposite the setting direction end, the cutting elements can be spaced further apart from each other because these cutting elements substantially only have a re-grooving function and merely serve to achieve an advantageous setting process of the screw in the substrate.
After the stamping, the recesses, preferably each, have a bottom section radially spaced apart from the outer circumference of the shank. The bottom section of the recess is advantageously spaced apart from the outer circumference of the shank such that during the welding of the cutting elements into the recess, only material of the thread formed on the shank is melted, and thus the shank as such is at most slightly exposed to thermal stress. The corrosion resistance of the shank of a screw of a corrosion-resistant material is largely preserved.
After the stamping, the recesses preferably have walls and the cutting element to be positioned in the recess is arranged so as to be spaced apart at least from one of the walls. This provides defined free spaces for removed material which improves the thread-forming process and thus facilitates the setting behavior of the screw of the invention.
According to a preferred embodiment of the screw, the cutting element is fixed by welding only on the bottom section of the recess and arranged so as to be spaced apart from both lateral walls of the recess. This freestanding arrangement of the cutting means in the recess, for example in combination with an ovaloid or spherical form of the cutting element, leads to an advantageous cutting behavior or thread-forming process and thus a better setting behavior. As a result, the setting time of a screw is reduced, on the one hand, and on the other hand, a lower torque is needed for setting the screw compared to the conventional thread-forming screws. This substantially contributes to the economy of the screw manufactured according to the method of the invention.
Alternatively, the cutting element is arranged directly adjacent to one of the walls of the recess and spaced apart from the other opposite wall, which provides a defined free space for removed material in front, behind or laterally next to the cutting element.
The compact cutting elements are preferably fixed in the recesses by means of electric resistance welding. This ensures a sufficiently firm bonding of the cutting elements in the recesses. In electric resistance welding, a high current is caused to flow between two bodies and at the same time the two bodies are pressed together by pressure. At the point of the highest electrical resistance, generally at the junction between the two bodies, high heat is generated which melts adjacent base material and thus establishes a connection between the bodies. Depending on the desired quality of the welding and the arrangement, different types of resistance welding methods may be used, such as the tip ignition method. Advantageously, methods should be selected that generate small zones influenced by the heat in the base material, especially the shank, and have short cycle times. As a result, the thread-forming screw is not only easy to manufacture, but the arrangement of the cutting elements has only a minor influence on the corrosion resistance of the screw.
Alternative welding methods for fixing the compact cutting elements in the recesses are friction welding, orbital welding, laser welding, or ultrasound welding.
The welding process for fixing the cutting elements in the recesses preferably takes place in a plurality of phases so that the compact cutting elements can be fixed on the thread by using welding programs especially optimized for this application. The welding programs control the time sequences and the level of the applied welding currents.
The phases of the welding process are preferably controlled via a closed loop, where the result influences the control variables of the welding equipment via feedback. Any interference variables and changes which may occur are taken into account and adjusted for by the control. A corresponding control of the welding parameters results in small zones influenced by the heat and therefore minimizes the degree of melting of the core and/or thread material, which is especially important for the corrosion resistance. The region-wise blending between the base material and cutting elements should be as low as possible, but sufficient to securely hold the cutting element in the thread and allow that the forces acting upon the cutting element during the setting of the screw can be diverted to the thread and thus the shank.
The welding process preferably comprises a first phase for heating the base material of the screw, a second phase for fixing the cutting elements in the recesses, and a third phase for cooling down the completed welding.
The first phase is the controlled heating phase that ensures, through locally restricted softening of the joining partners, in this case the screw material and the compact cutting elements, a secure bonding and partial preheating of the joining partners. The second phase is the controlled welding phase. The third phase is the controlled cool-down phase, which is especially advantageous for hard cutting elements, because it minimizes the formation of cracks through thermal shock during the cool-down.
The first phase, the second phase, and/or the third phase preferably consist of a plurality of partial phases, so that the heating, welding and cooling down can be optimally adjusted to the properties of the material of the joining partners.
The slope delta power to delta time is preferably steeper in the second phase than in the first phase so as to ensure a sufficient fixing of the cutting elements in the recesses in an advantageous manner.
The welding process preferably takes place with a defined contact pressing force of 5 N to 100 N coming from a holding device for the welded body and acting upon the welded body.
The rate of the defined contact pressing force is preferably 10 N to 60 N and especially 15 N to 40 N. The defined contact pressing force acting upon the cutting element secures that the correct position of the cutting element in the recess is precisely achieved and established and additionally ensures a sufficient fixing of the cutting elements in the recesses in an advantageous manner. If the cutting element is fixed in the recess by means of resistance welding, the flow of the welding current is improved by the contact pressing force acting upon the cutting element.
The defined contact pressing force preferably acts multi-axially, and the contact pressing force can advantageously act bi-axially or tri-axially. Due to the mobility of the holding device for the welded body (in the case of resistance welding a welding electrode, for example) caused by the respective multi-axiality, the compact cutting element, especially during the second and possibly during the third phase, may experience a relative motion with respect to the shank and thread of the screw which further improves the connecting quality of the fixing generated in the recess. For example, a bi-axially acting defined contact pressing force, on the one hand, acts radially upon the cutting element, which means in the direction of the shank, at 5 N to 100 N, and tangentially, on the other hand, which means in the direction of the thread, at 5 N to 100 N. The rate of the contact pressing force components of the defined contact pressing force does not necessarily need to be identical in radial and tangential direction.
A stamping device for stamping recesses in the thread of a screw is provided with at least one holding device for holding the screw during the stamping process and at least one stamping die for generating the recess in the thread of the screw.
The shaping portion of the at least one stamping die is formed according to the desired configuration of the recess to be produced. The at least one holding device holds the screw in the corresponding orientation so that the recess can be stamped in the desired position in the thread in a repeatable and precise manner.
A plurality of stamping dies is preferably provided for simultaneously generating a plurality of recesses. This reduces the cycle times for manufacturing the thread-forming screw compared to individually forming each recess. In this case, a stamping die can simultaneously form more than one recess in the thread. Alternatively, a plurality of stamping dies is provided, each of which forming one or a plurality of recesses in a feeding process in the thread of the screw.
If a plurality of stamping dies is provided or if one stamping die has a plurality of shaping portions, these are advantageously identically formed.
The at least one holding device and the at least one stamping die are preferably individually controlled. As a result, the retention forces acting upon the screw as well as the stamping forces may be set differently. Therefore, deformations of the screw or the thread and especially a warping of the shank can be prevented.
The invention is described in greater detail below according to exemplary embodiments.